Polymer blend compositions for use in engineering applications should exhibit a combination of physical properties such as heat resistance and flame retardancy, good impact strength and good modulus. Additionally, the blend compositions should exhibit good melt flow properties which facilitate processing and molding of the blend compositions. Polycarbonates are popular blend components owing to their toughness and relatively high softening temperatures. However, owing to their relatively poor melt flow characteristics, polycarbonates are often blended with one or more additional polymers to improve their melt flow properties. Examples of such blend compositions include the Grabowski U.S. Pat. No. 3,130,177 which discloses blends of polycarbonates with polybutadiene, styrene, acrylonitrile graft polymers and the Grabowski U.S. Pat. No. 3,162,695 which discloses blends of polycarbonates with butadiene-styrene, methyl methacrylate, styrene graft copolymers. The Parsons U.S. Pat. No. 4,886,855 discloses blends of polycarbonate, ABS resin and styrene polymers which exhibit a good combination of physical properties. The Liebig et al U.S. Pat. No. 4,205,140 also discloses a thermoplastic molding composition comprising a blend of a polycarbonate, a diene rubber graft polymer such as ABS, and a styrene polymer.
The Gaggar copending application Ser. No. 07/369,157 filed Jun. 21, 1989, discloses thermoplastic compositions comprising a blend of polycarbonate and a grafted ABS resin, which compositions further contain a polyalkylmethacrylate as a weld line strength enhancer. In the molding of parts from thermoplastic materials, it is common to use a mold design in which molten plastic flowing in one direction meets molten plastic flowing from another direction as the mold fills. Where the two masses meet, the streams fuse together and form a bond at the juncture. The zone where the materials join is commonly referred to as the weld line or the knit line. The tensile strength of a molded part at the weld line is generally lower than in other sections of the part. The compositions disclosed in the copending Gaggar application, the contents of which are incorporated herein by reference, overcome this disadvantage in that the inclusion of the polyalkylmethacrylate improves the weld line strength of parts molded therefrom.
It is also desirable that blend compositions of polycarbonate polymers and ABS resins exhibit flame retardant and fire retardant properties. In the past, these properties have been provided by the addition of one or more conventional inorganic and/or organic flame retardant or fire retardant additives. These additives are added in varying amounts and provide varying degrees of flame and fire retardant properties. In many instances however the flame and fire retardant additives disadvantageously affect other physical properties of the compositions.
Accordingly, a need exists for improved compositions of polycarbonate and ABS resins which exhibit satisfactory flame retardant properties in combination with good physical properties, including weld line strength.